Printing devices are peripherals for printing characters and/or graphics on papers or other kinds of printing media. Generally, the printing devices are classified into two types: ordinary printing devices and thermal transfer printing devices. The configurations of the thermal transfer printing devices are substantially identical to those of the ordinary printing devices except for the printing way. For example, the ordinary printing device supplies ink or toner onto a paper. Whereas, a thermal transfer printing device has a thermal transfer printing module for outputting the image. The thermal transfer printing module has a thermal print head (TPH) to heat a coating and allow the coating to be adsorbed on a thermal transfer paper, so that the image is printed out. The widely-used thermal transfer printing devices include for example faxing machines, POS (Point of Sale) printers and barcode printers.
FIG. 1 is a schematic side view illustrating a conventional thermal transfer printing device. As shown in FIG. 1, the conventional thermal transfer printing device 1 comprises a casing 10, a thermal transfer paper 11, a thermal transfer printing module 12, a transport roller assembly 13, and an ejecting roller assembly 14. Generally, the thermal transfer paper 11 is wound as a paper roll for storage, and disposed within the casing 10. The thermal transfer paper 11 has a first end 111 in contact with the transport roller assembly 13. The transport roller assembly 13 is contacted with the thermal transfer paper 11 for transporting the thermal transfer paper 11 to the thermal transfer printing module 12. The thermal transfer printing module 12 is arranged downstream of the transport roller assembly 13 for printing an image on the thermal transfer paper 11. The thermal transfer printing module 12 comprises a thermal print head 121 and a print roller 122. The thermal print head 121 is used for heating a coating (not shown) and allowing the coating to be adsorbed on the thermal transfer paper, thereby printing out the image. The print roller 122 is disposed under the thermal print head 121. The print roller 122 is used for transporting the thermal transfer paper 11 and pressing the thermal transfer paper 11. Consequently, the thermal transfer paper 11 is smoothly transported across the region under the thermal print head 121 while maintaining the printing performance. The ejecting roller assembly 14 is arranged downstream of the thermal transfer printing module 12 for ejecting the thermal transfer paper 11 out of the casing 10.
A process of printing the thermal transfer paper 11 by the thermal transfer printing module 12 will be illustrated as follows. Firstly, the first end 111 of the thermal transfer paper 11 is transported across and printed by the thermal transfer printing module 12. After the first end 111 of the thermal transfer paper 11 has been printed, the first end 111 of the thermal transfer paper 11 is continuously transported to the ejecting roller assembly 14, and a middle segment 112 of the thermal transfer paper 11 is continuously printed by the thermal transfer printing module 12. At this moment, the first end 111 of the thermal transfer paper 11 is transported by the ejecting roller assembly 14, and the middle segment 112 of the thermal transfer paper 11 is transported by the print roller 122. Moreover, the middle segment 112 of the thermal transfer paper 11 is transported by the print roller 122 at a first speed, and the first end 111 of the thermal transfer paper 11 is transported by the ejecting roller assembly 14 at a second speed. Ideally, the transporting speeds of the print roller 122 and the ejecting roller assembly 14 are equal. In practice, since the components of the thermal transfer printing device 1 have respective allowable tolerances, the accumulated allowable tolerance of the combined components will be increased. Consequently, the thermal transfer paper 11 is transported by ejecting roller assembly 14 at the second speed, which is slightly slower than the first speed.
Since the speed of transporting the first end 111 of the thermal transfer paper 11 is slower than the speed of transporting the middle segment 112 of the thermal transfer paper 11, the portion of the thermal transfer paper 11 between the first end 111 and the middle segment 112 is readily upturned during the printing process (see FIG. 1). Since the thermal transfer paper 11 is upturned, the thermal transfer paper 11 fails to be smoothly transported. Under this circumstance, the printing performance of the thermal transfer printing device 1 is deteriorated.
Therefore, there is a need of providing a thermal transfer printing device with enhanced printing performance.